It is frequently desired to form a functionalized monolayer over a substrate surface. FIGS. 1 and 2 describe a general scheme by which a functionalized monolayer can be formed over a surface. Referring initially to FIG. 1, a precursor 10 suitable for utilization in forming a monolayer is illustrated. Precursor 10 comprises an attaching group 12 which ultimately is utilized for forming an attachment to the substrate surface. The attachment between attaching group 12 and the surface can comprise a covalent bond. Accordingly, attaching group 12 of precursor 10 can comprise suitable leaving groups which are displaced when attaching group 12 interacts with atoms associated with the surface so that bonds are formed to connect the attaching group to the surface.
Attaching group 12 is connected to a linker 14, which extends to a functional group 16. Linker 14 (which can also be referred to as a spacer) can be a hydrocarbon chain, and is shown comprising 5 carbon atoms. The length of spacer 14 can be varied depending on a desired distance between functional group 16 and attachment group 12.
Functional group 16 comprises a desired functionality. For instance, if a monolayer is ultimately to be utilized for extracting mercury or other heavy metals, functional group 16 can comprise thiols as described in Feng, X.; Fryxell, G. E.; Wang, L. Q.; Kim, A. Y.; Liu, J.; Kemner, K. Science, 1997, 276, 923-926(Feng et al, 1997).
Referring to FIG. 2, a substrate 18 is provided, and a surface 20 of the substrate is exposed to a plurality of the precursors 10 (only some of which are labeled). The precursors bind to upper surface 20 through attachment group 12 to form a monolayer 22 over surface 20. Such monolayer is effectively a coating over surface 20, and an outer periphery of such coating corresponds to functional groups 16. The monolayer 22 can be referred to as a monolayer of functionalized material formed over substrate surface 20, and the process of forming monolayer 22 over the substrate of surface 20 can be referred to as functionalizing of the surface.
Several applications are known in which it is desired to form a functionalized monolayer over a substrate surface. Such applications include functionalizing surfaces of porous materials (for example, mesoporous ceramics and zeolites), and non-porous materials. However, there remains a need to develop improved methodologies for functionalizing substrate surfaces.
With respect to the mesoporous materials, significant efforts have been made to develop methodologies suitable for functionalizing surfaces of the materials, and particular efforts have been made to find methodologies suitable for functionalizing surface regions extending within pores of the mesoporous materials. Since their unveiling in 1992, mesoporous ceramics have inspired substantial interest, especially by adding self-assembling monolayer compounds to the surface(s) of the mesopores. By varying the terminal group of the self-assembling monolayers, various chemically functionalized materials have been prepared. A mesoporous material is defined as a material, usually a catalytic material, having pores with a diameter or width range of 2 nanometers to 0.05 micrometers.
Exemplary methods of forming and using self-assembling monolayer(s) on mesoporous materials are described in the International Application Publication WO 98/34723. The self-assembling monolayer(s) are made up of a plurality of assembly molecules each having an attaching group. For attaching to silica, the attaching group may include a silicon atom with as many as four attachment sites, for example; siloxanes, silazanes, and chlorosilanes. Alternative attaching groups include metal phosphate, hydroxamic acid, carboxylate, thiol, amine and combinations thereof for attaching to a metal oxide; thiol, amine, and combinations thereof for attaching to a metal; and chlorosilane for attaching to a polymer. A carbon chain spacer or linker extends from the attaching group and has a functional group attached to the end opposite the attaching group, as described in FIGS. 1 and 2.
Methods of attaching and constructing a self-assembling monolayer on a mesoporous material can involve solution deposition chemistry in the presence of water. More specifically, as reported in Feng et al, 1997; and Liu, J.; Feng, X.; Fryxell, G. E.; Wang, L. Q.; Kim, A. Y.; Gong, M. Adv. Mat. 1998, 10, 161-165 (Liu et al., 1998), a synthetic protocol to prepare monolayers of MPTMS (mercaptopropyl trimethoxysilane) within the pores of MCM-41 can involve a 1-hour hydration step, followed by a 6-hour silanation step in refluxing toluene. At this stage, the silane coverage is limited to approximately 3.6-4.0 silane molecules/nm2 (this surface density is not enhanced by either extending the reaction time or increasing silane concentration). Following the silanation with a 2-3 hour azeotropic distillation drives the equilibria through the removal of reaction by-products, and increases this surface density to 5.0-5.2 silanes/nm2. This surface density is representative of typical silane-based monolayers. The monolayer coated mesoporous product is then isolated by filtration, washed extensively and dried for several days. The overall procedure typically takes about 10 hours of laboratory preparation time and 1-10 days of drying time. The reaction time is driven by the kinetics of getting the self-assembling molecules into the mesopores, and getting the water and any other solvent out of the mesopores.
The product obtained exhibits a maximum of 40% of the monolayer silicon atoms fully crosslinked for maximizing monolayer stability. Ideally, 100% of the silicon atoms would be fully crosslinked. Full crosslinking is having three of the four bonding sites linked to another silicon atom via an oxygen atom, with the fourth linked to the functional group terminated linker (typically a hydrocarbon chain). The crosslinking can be impeded by the presence of “dangling” hydroxyl groups. Such hydroxyl groups represent a defect in the crosslinking of the monolayer, and thus place a practical upper limit on the number of silicon atoms that are fully crosslinked.
Thermal curing of silane monolayers can be utilized to increase crosslinking. Typical thermal curing (ca. 150° C.) of a silane monolayer creates a terminal to internal silane ratio of 1:2, corresponding to about 60% to 65% of attaching molecules (silicon) being fully crosslinked.
It is desirable to develop methods of forming self-assembling monolayers in which a high fraction of the assembly atoms are fully crosslinked. It is desirable for the methods to be applicable for a diverse range of substrates, including mesoporous materials. It is desirable to have a high surface density of monolayer coverage. It is also desirable to develop methods of forming the monolayers that are rapid and economical.
It can be difficult to adequately diffuse monolayer precursors (typically organic molecules) into the small pore channels of mesoporous materials during functionalizing of the mesoporous materials. In the last few years, both post-silanization and in-situ deposition have been successfully applied to mesoporous materials, in which the pore diameter is usually larger than 2 nm. The mesoporous materials (usually synthesized using surfactant micelles as templates) have very uniform pore sizes. Because of their high surface area and the open pore channels; functionalized mesoporous materials have been investigated for many adsorption and catalysis applications. However, due to the large pore size and the amorphous nature of the materials, these materials appear unlikely to find application as size selective catalysts.
A zeolite is any one of a family of hydrous aluminum silicate minerals whose molecules enclose cations of sodium, potassium, calcium, strontium, or barium, or a corresponding synthetic compound. Zeolites are typically used as molecular filters and ion-exchange agents. Compared to the mesoporous materials, the diffusion of organic molecules in zeolites is severely hindered by the small pore size. Deposition of silanes on the exterior surface is therefore greatly favored over silanation of internal surfaces. Heretofore, it had been believed that introducing organic functional groups to the internal pore surfaces of commercial zeolites to produce size selective microporous catalysts could not be achieved due to the size of the pores.